Thermal Break Shear Wall at Sage Green
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For this 5-unit entry-level, zero-net-energy subdivision in Washington County, Oregon, the builder used an innovative “Thermal Break Shear” (TBS) wall assembly with rigid foam insulation between the lumber framing and plywood sheathing in an otherwise conventional light-frame wall assembly. The Code Official required proof the proposed shear wall assembly would be capable to resist code level seismic forces, so the builder contracted with Oregon State University to perform destructive seismic testing, which demonstrated not only that the assembly complies with structural code, but surprisingly, TBS wall is significantly more resilient in an earthquake than a conventional wall.
Green One Construction, with support from ongoing research and development programs at the Oregon Department of Energy (ODOE) and Energy Trust of Oregon (ETO), built five net-zero energy homes in the Sage Green subdivision. Ben Walsh of Green One Construction led the project team, coordinating the efforts of the developer, subcontractors and R&D partners to meet the requirements of the Jurisdiction. In the course of investigating a wall assembly with exterior foam insulation, the collaborative team innovated a simple, low-cost alternative with 1.25” of rigid foam between the lumber and ply sheathing. They realized this could deliver significant energy savings through a complete thermal break and airtight building envelope, while resolving other challenges in constructing a high performance assembly.
In August 2009, Walsh presented the TBS wall assembly to Dr. Kofi Nelson-Owusu, Senior Structural Engineer and Supervising Plans Examiner at the Washington County, Oregon, LUT - Building Services. Dr. Nelson-Owusu required testing to demonstrate the strength of the wall assembly. Tests were performed by Oregon State University's Knudsen Wood Engineering Laboratory in Corvallis in September 2009. Surprisingly, those tests showed that the TBS assemblies actually increases the flexibility of the wall, delivering higher lateral load capacity than a conventional wall. Dr. Nelson-Owusu accepted the test results and structural engineering, and Washington County issued building permits on January 25, 2010.
Of particular interest to anyone who might seek to use a TBS assembly, on June 1, 2013, the International Code Council Evaluation Services (ICC-ES) published Evaluation Services Report 2586. Beginning in 2015, building regulators at the City of Portland have acknowledged that ICC-ESR 2586 establishes that a TBS wall assembly with up to 1” of foam meets requirements for lateral bracing under International Residential Code (IRC) Table 602.10.2, Method WSP, and confers prescriptive approval on such an assembly with no engineering required.
|Code Requirement||Compliance Path|
|2010 Oregon Structural Specialty Code: Section 1403 specifies requirements for exterior wall assemblies, including structural load resistance required by Chapter 16.||To get the TBS wall approved, the code official required seismic lateral strength testing. Testing was performed, and results were provided to the County, who approved the wall as an alternative method.|
In support of Green One's effort to develop the net-zero energy model for the homes, ODOE and the Energy Trust of Oregon initially suggested a high performance wall assembly with rigid foam sheathing added to the outside of the wall assembly to create a continuous thermal break. Yet, this method complicates numerous other related processes and is costly.
For example, adding rigid foam exterior to the otherwise conventional wall meant the foam would most likely be installed by the siding contractor after the walls were “tilted up” rather than being installed by the framer while the walls were still flat on the floor. Building walls flat is a common practice that makes the process safer and more economical. Exterior foam was a complicated prospect with potential for mistakes and guaranteed higher cost.
Design / Build Process
In the course of working through challenges with exterior foam, the project team had a breakthrough idea of having the framer install 1.25” EPS foam board between the lumber frame and plywood sheathing instead of on the outside, which resolved many of the problems with the earlier wall concept. In the builder’s own words, “I was just trying to get a thermal break into the wall assembly at the least cost. Having the framers put rigid foam between the frame and the ply looked like the quickest, simplest way: work is on flat walls, and it doesn't disrupt established framing or other envelope practices.”
With this solution they could assemble the layers together on top of the framing, and then tip the wall up as a complete panel. Walsh notes “The one exception is transference across floor lines: we had to use threaded rod and back-to-back hold-downs in the frame cavities instead of the typical flat strap on the outside.”
To prove that inserting foam between the framing and shear panel would not degrade the lateral capacities of the assembly, structural engineer Scott Nyseth was engaged to prepare structural calculations for the wall, who at the time worked for Miyamoto International. For the system to work, Nyseth said sheathing would need to be nailed every 3” on the perimeter, rather than the standard 6”. Then the team contracted with Dr. Rakesh Gupta at Oregon State University’s Knudsen Wood Engineering Laboratory in Corvallis to perform destructive testing of a series of wall panels of various foam thicknesses. The Knudsen team performed ASTM-E72/CUREE-style cyclic lateral load testing (ie: commercial code standard), in conformance with ICC Evaluation Service Approval Criterion 130, on September 14, 2009.
The test report summarizes the results as follows:
“The graph below compares hysteresis loops, force curves, from the tests of a conventional wall, and a [TBS] wall. The curves describe wall panels bolted into a test rack, being pushed and pulled increasingly out-of-square by a test cylinder [simulating the cyclical racking of a building in an earthquake]. The blue line shows a conventional assembly reaching its capacity at about 1-3/4” of deflection, under a 5,600-lb load, and suffering catastrophic failure. The green line shows a [TBS] wall continuing to resist deflection with increasing strength as the test reaches its protocol maximum deflection of 5”.
These results showed that the TBS wall's intermediate foam increases the flexibility of the assembly, delivering a higher lateral load capacity than a conventional wall, and also making the wall much more resilient in the face of the racking motion typical of seismic events.
The key to this surprising result was the additional flexibility that results from the nails connecting plywood to the framing through the compressible intermediate foam layer. When a conventional wood-to-wood wall assembly is racked back and forth, the shear strength of the nails destroys the plywood. In a TBS wall, the gap created by the foam takes the nails out of shear and puts them in a bending condition, thereby dissipating the energy of the cyclic lateral loading.
As a result, Dr. Nelson-Owusu accepted the test results and the engineering based on those results. Washington County issued building permits on January 25, 2010 and the five homes were completed and approved for occupancy on August 10, 2010.
Christopher Dymond, now a Senior Project Manager at the Northwest Energy Efficiency Alliance was the ODOE Project Manager that worked with Green One to develop the high-performance wall in 2009. Dymond and Walsh collaborated on a detailed project report published by the Northwest Energy Efficiency Alliance on February 22, 2016. That report is the primary source of information for this case study.
|“Thermal Break Shear Wall: Report #E 16-296”, by Miter Construction Management LLC. 2016: Northwest Energy Efficiency Alliance Portland Oregon.||ICC-ES Evaluation Report ESR-2586. Report Holder: APA American Engineered Wood Association. Published June 1, 2013|
|Dunn, Glandics, and Van Den Wymelenberg. "Net Zero Energy Homes: Sage Green." University of Idaho, Integrated Design Lab: April 1, 2011||IECC Compliance Options for Wood-Frame Wall Assemblies|
Cost / Benefit
The Northwest Energy Efficiency Alliance Report #E16-296 includes a cost analysis comparing several different wall assemblies. It concludes that “the 2x6 TBS wall assembly consistently offers better ‘bang for the buck’” as a strategy to improve thermal performance without disrupting framing contractors’ familiar construction process. Compared to standard 2x6 framing, the 2x6 TBS Wall yields 32% lower heat loss (i.e. U-value) and 47% increase in insulative R-value. These gains were achieved for about 65% less [incremental] cost than the exterior foam assembly which performs only slightly better that the TBS wall. Labor to install the additional layer of foam added just $0.25 / square foot; and materials added $0.50 / square foot, adding up to an incremental cost increase of $1,200 per home.
Building the wall requires double the number of nails which increases heat loss by 2% compared to the exterior foam wall. Yet because of the nails it also achieves greater airsealing of the assembly, stopping air leaks due to the slight compression of foam between the frame and sheathing.
|Builder: Ben Walsh, Principal Green One Construction||Consultant: Christopher Dymond, Senior Project Manager Northwest Energy Efficiency Alliance||Structural Engineer: Scott Nyseth, S.E., Engineer Miyamoto International Engineering|
|Approving Offcial: Dr. Kofi Nelson-Owusu, Ph.D., S.E., P.E., Building Engineer / Supervisor Washington County, Oregon Building Services||Approving Offcial: John McAllister, Building Official Washington County, Oregon Building Services|